1. Field of the Invention
The present invention relates to an expanded mesh sheet that is preferable for use as a battery collector, and more particularly to a method and apparatus for manufacturing an expanded mesh sheet with which a fine reticulated configuration can be formed in a thin metal sheet for realizing smaller, thinner, and higher capacity batteries, and to a battery using this expanded mesh sheet.
2. Description of the Related Art
As portable instruments such as portable telephones have become smaller, thinner, and lighter in weight, the batteries used as a source of power by such instruments have been required to have higher capacities, as well as to be smaller and lighter. FIGS. 13 and 14 show the constitution of a polymer electrolyte secondary cell, a type of battery developed in response to these requirements. This battery comprises a laminated electrode 4, of positive electrode plates 1 and a negative electrode plate 2 laminated together with separators 3 therebetween, held within an outer case 7 comprising a laminated sheet. As shown in FIG. 14, which shows a cross sectional view of FIG. 13 at line XIV--XIV, the above-mentioned positive electrode plate 1 is formed of positive electrode active material 1b coated on a positive electrode collector 1a; the above-mentioned negative electrode plate 2 is formed of negative electrode active material 2b coated on a negative electrode collector 2a. The positive electrode plate 1 and negative electrode plate 2 are layered together with separators 3, consisting of solid electrolyte material therebetween, and sealed along with liquid electrolyte within an outer case 7, comprising a pair of laminated sheets heat fused along their perimeters at seal portions P.sub.1, P.sub.2, P.sub.3. The positive electrode lead 8 is connected to the lead connecting portions 1c, 1c each formed on the two positive electrode collectors 1a, 1a and the negative electrode lead 9 is connected to the lead connecting portion 2c formed on the negative electrode collector 2a. The positive electrode lead 8 and negative electrode lead 9 are insulated from each other with an insulating sheet 6 and drawn out to the outside of the outer case 7 so as to be used as battery connection terminals for the positive and negative electrodes of the battery.
The above-mentioned positive electrode plate 1 and negative electrode plate 2 are manufactured as follows. The positive electrode active material, prepared as a paste, is applied on an expanded mesh sheet of aluminum that will constitute the positive electrode collector 1a, which is then dried and rolled to form a positive electrode sheet. The positive electrode place is cut to a prescribed form and size from this positive electrode sheet thus obtained wherein the positive electrode active material 1b is adhered to a prescribed thickness on the positive electrode collector 1a. Also, the negative electrode active material, prepared as a paste, is applied on both surfaces of an expanded mesh sheet of copper that will constitute the negative electrode collector 2a, which is then dried and rolled to form a negative electrode sheet. The negative electrode plate is cut to a prescribed form and size from this negative electrode sheet thus obtained wherein the negative electrode active material 2b is adhered to a prescribed thickness on both surfaces of the negative electrode collector 2a. As shown in FIG. 13, the positive electrode plate 1 and negative electrode plate 2 are cut from the positive electrode sheet and negative electrode sheet respectively, such that the lead connecting portion 1c protrudes from the positive electrode collector 1a at a position offset from the center line, and such that the lead connecting portion 2c protrudes from the negative electrode collector 2a at a position offset from the center line opposite from the lead connecting portion 1c of the positive electrode collector 1a. The aluminum positive electrode lead 8 is joined to the lead connecting portion 1c of the positive electrode collector 1a, and the copper negative electrode lead 9 is joined to the lead connecting portion 2c of the negative electrode collector 2a, respectively, at welding points S using resistance welding or ultrasonic welding.
In order to satisfy the requirement that a battery be small, light, and have higher capacity, the expanded mesh sheet used as a collector must be thin, with a fine mesh grid, and yet have the strength to withstand the tensile force applied during manufacture. Moreover, the mesh sheet must be superior in binding properties to the active material and collecting properties.
In the collector, the electrodes of same polarity are connected to each other at the lead connecting portions, and the lead is further connected thereto. Therefore, the lead connecting portions are required to have better welding properties as the number of laminated positive and negative electrodes increases. However, in collectors wherein an expanded mesh sheet with a high rate of openings is used, the lead connecting portions tend to have poor welding properties, and low bonding strength and conductivity between collectors and leads.
The above-mentioned expanded mesh sheet is manufactured by pulling a metal sheet, wherein slits have been formed in a zigzag pattern, in a direction perpendicular to the orientation of the slits, thereby opening the slits to form a lozenge-shaped reticulated configuration.
In other words, as shown in FIG. 15A, a multiplicity of slits a are formed intermittently and parallel to each other in the direction in which the metal sheet A extends. The slits a are arranged in a zigzag pattern, with the parallel and adjacent positions being offset in the direction in which the metal sheet A extends; nodes b are formed between intermittent slits a, a. Furthermore, as shown in FIG. 15B, bulges c are formed by plastic deformation at positions sandwiched between slits a, a juxtaposed in a widthwise direction, protruding from both surfaces of the metal sheet alternately in opposite directions. Such a metal sheet, wherein slits a, nodes b, and bulges c have been formed, is pulled widthwise as shown in FIG. 16 to attain an expanded mesh sheet having a mesh grid structure, wherein the slits a, a are opened thus forming lozenge-shaped openings surrounded by linear lattice bars d connected by nodes b.
Current methods for manufacturing this type of mesh sheet include those using a rotary system and those using a reciprocating system.
FIG. 17 shows an example of a rotary-type apparatus for manufacturing expanded mesh sheets. This apparatus includes a pair of rollers 100, 100 which is constructed such that a plurality of disk-shaped cutters 31, that are provided with raised portions 32 for forming the above-mentioned bulges c on the periphery, are superposed coaxially at intervals approximately equal to the thickness of the disk-shaped cutters 31. The rollers 100, 100 are disposed opposite to each other with their axes being parallel and their positions in the axial direction being offset by the thickness of the disk-shaped cutters 31. Blades for forming the slits a in the direction in which the metal sheet A is supplied and in an area between the disk-shaped cutters 31 of one roller 100 and the disk-shaped cutters 31 of the other roller 100, are formed on both edges of each disk shaped cutter 31. Recessed portions 33 for interrupting the formation of the slits a and for forming the nodes b are formed at a prescribed pitch on the blades in the direction of the circumference of the disk-shaped cutters 31. By supplying the metal sheet A between the rollers 100, 100 and rotating the rollers 100, 100 around their respective axes, the slits a connected with the nodes b are formed in the metal sheet A, as well as the bulges c protruding in mutually opposite directions are formed at positions where slits a, a, are juxtaposed with each other, as shown in FIGS. 15A and 15B.
One disadvantage of such rotary system is that the disk-shaped cutters 31 need to have at least a certain minimum thickness in order to ensure the strength of the disks, wherefore there are limits to the thinness and fineness of the cutters. For this reason, the above-mentioned reciprocating system is more appropriate for manufacturing expanded mesh sheets with a fine mesh grid.
In the method of manufacturing expanded mesh sheets with a reciprocating system, slits j are formed intermittently and widthwise in an elongated sheet 55 in such a manner that the slits j are arranged in a zigzag pattern offset in the direction of the orientation of the slits as shown in FIG. 19 by a pair of upper and lower plate cutters 53, 54 as shown in FIG. 18, one row at a time. The cutters 53, 54 include raised portions as part of slit forming blades, with which bulges k are formed at the same time that the slits j are formed. The elongated sheet 55, in which the slits j and bulges k have been formed in a zigzag pattern, is pulled in a direction lengthwise to the elongated sheet as shown in FIG. 19, whereupon the slits j and bulges k are spread out and an expanded mesh sheet having a lozenge-shape mesh is attained.
The above-described method for manufacturing expanded mesh sheets with a reciprocating system has problems of high costs, low productivity, and slow working speed because the slits j are made one row at a time. Also, it is preferable that the ends of the expanded mesh sheet include solid portions where a mesh is not formed in order to ensure the strength to withstand processes of applying active material to and rolling the thin expanded mesh sheet and in order to form lead connecting portions for connecting leads to the collectors. However, with the reciprocating system as shown in FIG. 18, a mesh structure has to be formed on the entire surface of the mesh sheet because of the specific expanding method, wherefore it was impossible to manufacture an expanded mesh sheet provided with solid portions which has no reticulated configuration.